With the recent rapid development of portable and cordless electronic devices such as audio-visual (AV) devices and personal computers, there is an increasing demand for secondary batteries or batteries having a small size, a light weight and a high energy density as a power source for driving these electronic devices. Under these circumstances, lithium ion secondary batteries having advantages such as a high charge/discharge voltage and a large charge/discharge capacity have been noticed.
Hitherto, as positive electrode active substances useful for high energy-type lithium ion secondary batteries exhibiting a 4 V-grade voltage, there are generally known LiMn2O4 having a spinel structure and LiMnO2, LiCoO2, LiCo1−xNixO2 and LiNiO2 having a rock-salt type structure, or the like. Among these active substances, LiCoO2 is more excellent because of a high voltage and a high capacity thereof, but has the problems such as a high production cost due to a less amount of a cobalt raw material supplied, and a poor environmental safety upon disposal of cells obtained therefrom. In consequence, there have now been made earnest studies on lithium manganate particles with a spinel type structure (basic composition: LiMn2O2; this is similarly applied to the subsequent descriptions) which are produced by using, as a raw material, manganese having a large supply amount, a low cost and a good environmental compatibility.
As is known in the art, the lithium manganate particles may be obtained by mixing a manganese compound and a lithium compound at a predetermined ratio and then calcining the resulting mixture at a temperature of 700 to 800° C.
When using the lithium manganate particles as a positive electrode active substance for lithium ion secondary batteries, the resulting cell has a high voltage and a high energy density, but tends to be deteriorated in charge/discharge cycle characteristics. The reason therefor is considered to be that when charge/discharge cycles are repeated, the crystal lattice is expanded and contracted owing to desorption and insertion behavior of lithium ions in the crystal structure to cause change in volume of the crystal, which results in occurrence of breakage of the crystal lattice or dissolution of Mn in an electrolyte solution.
At present, in the lithium ion secondary batteries using lithium manganate particles, it has been strongly required to suppress deterioration in charge/discharge capacity due to repeated charge/discharge cycles, and improve the charge/discharge cycle characteristics, in particular, under high-temperature and low-temperature conditions.
In order to improve the charge/discharge cycle characteristics of the cells, it is required that the positive electrode active substance used therein which comprises the lithium manganate particles has an excellent packing property and an appropriate size, and further is free from elution of Mn therefrom. To meet the requirements, there have been proposed the method of suitably controlling a particle size and a particle size distribution of the lithium manganate particles; the method of obtaining the lithium manganate particles having a high crystallinity by controlling a calcination temperature thereof; the method of adding different kinds of elements to the lithium manganate particles to strengthen a bonding force of the crystals; the method of subjecting the lithium manganate particles to surface treatment or adding additives thereto to suppress elution of Mn therefrom; or the like.
Conventionally, it is known that aluminum as one of the different kinds of elements is incorporated in the lithium manganate particles (Patent Documents 1 to 6). In addition, it is known that a sintering aid having a melting point of not higher than 800° C., in particular, boron oxide, boric acid, lithium borate or ammonium borate, is added to the lithium manganate particles upon calcination of the particles to attain the effects by addition of the sintering aid (Patent Documents 7 to 11).
More specifically, there are respectively described the method of incorporating a Ca compound and/or an Ni compound as well as an Al compound in the lithium manganate particles (Patent Document 1); the method of incorporating Al in the lithium manganate particles in which positions of peaks of respective diffraction planes as observed in X-ray diffraction analysis thereof are defined (Patent Document 2); the method of incorporating a different kind of element such as Al in the lithium manganate particles and conducting the calcination of the lithium manganate particles at multiple separate stages (Patent Document 3); lithium manganate which is obtained by incorporating a different kind of element such as Al into lithium manganate particles, and has a specific surface area of 0.5 to 0.8 m2/g and a sodium content of not more than 1000 ppm (Patent Document 4); lithium manganate which is obtained by incorporating a different kind of element such as Al into lithium manganate particles, and has a half value width of (400) plane of not more than 0.22° and comprises crystal particles having an average particle diameter of not more than 2 μm (Patent Document 5); lithium manganate which is obtained by incorporating a different kind of element such as Al into lithium manganate particles, and has a crystallite size of not less than 600 Å and a lattice distortion of not more than 0.1% (Patent Document 6); lithium manganate which is obtained by heat-treating a lithium compound, manganese dioxide and a boron compound at a temperature of 600 to 800° C. (Patent Document 7); lithium manganate particles into which an element whose oxide has a melting point of not higher than 800° C. and a fluorine compound are incorporated (Patent Document 8); a lithium secondary battery comprising boron-containing lithium manganate and lithium nickelate in which the lithium nickelate is present in an amount of 5 to 20% by weight based on the lithium manganate (Patent Document 9); and lithium manganate comprising lithium tetraborate which is calcined at a temperature of 700 to 850° C. (Patent Document 10).
Patent Document 1: Japanese Patent Application Laid-Open (KOAKI) No. 2000-294237
Patent Document 2: Japanese Patent Application Laid-Open (KOAKI) No. 2001-146425
Patent Document 3: Japanese Patent Application Laid-Open (KOAKI) No. 2001-328814
Patent Document 4: Japanese Patent Application Laid-Open (KOAKI) No. 2002-33099
Patent Document 5: Japanese Patent Application Laid-Open (KOAKI) No. 2002-316823
Patent Document 6: Japanese Patent Application Laid-Open (KOAKI) No. 2006-252940
Patent Document 7: Japanese Patent Application Laid-Open (KOAKI) No. 8-195200 (1996)
Patent Document 8: Japanese Patent Application Laid-Open (KOAKI) No. 2001-48547
Patent Document 9: Japanese Patent Application Laid-Open (KOAKI) No. 2002-170566
Patent Document 10: Japanese Patent Application Laid-Open (KOAKI) No. 2005-112710